A five-year continuation and extension of the Virginia Study of Adolescent Behavioral Development is proposed. A population-based random sample of 1366 twin pairs aged 8-16 years and their parents has been ascertained and studied on two occasions by semi-structured psychiatric home interview (the CAPA), questionnaire, and teacher ratings. Major outcome variables in the children include attention-deficit disorder/hyperactivity, oppositional behavior, conduct disorder, anxiety and depression. Parental anxiety, depression, anti-social personality, alcoholism, phobia and panic disorder are also assessed by standardized home psychiatric interview. Measurements of the home environment and the differential environment within twin pairs are secured. In the next five years it is proposed to 1) follow up with the same protocol the existing twins at as they reach approximately 11-12,13-14 and 15+ years of age and 2) screen by telephone for hyperactivity (8 years, N=5271 pairs) and depression (16 years, N=2896 pairs) the twin populations of Virginia and North Carolina, and follow-up with the identical protocol any (N=c.1300) family in which either or both twins are in the upper decile of symptomatology for ADDH or depression. Analysis of the combined random and enriched longitudinal data set on identical and non-identical twins, combined with assessments of parents and specific environmental risk factors will answer a number central questions concerning the etiology and developmental trajectory of major disorders in adolescence. For example: "How much do the effects of genes, the shared family environment and the unique experiences of the individual lead to behavioral disorders of adolescence?"; "What is the effect of parental psychopathology on the behavior of their children?"; "What particular features of the environment account for significant non-genetic risk? ";"Do these specific environmental risk factors tend to have a more serious impact in genetically high-risk children?" (Genotype x environment interaction); "Do some genetic differences in behavior tend to attract particular kinds of environmental treatment?" (Genotype-environment correlations); "Do boys and girls differ for the genetic and environmental factors which affect the risk and developmental trajectory of adolescent behavior?" (genotype x sex interaction); "Are some behavioral disorders of adolescence etiologically heterogeneous?"; "Do individuals in whom more than one disorder is expressed (comorbidity) reflect distinct etiological and developmental pathways?"; "To what extent are adolescent behavioral disorders better conceived categorically and to what extent are they best conceived dimensionally?"; "What factors, genetic or environmental affect the timing of onset and developmental trajectory of behavioral disorders of adolescence?"